20-Year Prospect
Well, in twenty years we're definitely still going to need people to research new immunological cures, test for allergies, and to generally try to combat the spread of diseases. But there could also be some complications to the sunny outlook for the immunology field.
For one thing, there's a whole bunch of hoopla surrounding our increased resistance to antibiotics (source). The diseases that antibiotics help to cure are also able to mutate; as they continue to resist new drugs invented to stop them, the diseases threaten to continue infecting people no matter what sorts of cures are cooked up. It's hard to say how it'll all shake out, but the situation has many scientists worried.
That said, we're going to need our best and brightest to continue combating the situation—no matter what. Immunologists will be important for that, and job prospects for the various sectors of immunology will continue to grow, even if at a sluggish rate (source).
If you're worried about whether you might go through all the long, hard work of studying to be an immunologist, only to have the job not exist anymore once you get there, you can rest easy. If you dedicate yourself to the study of medicine to the extent you'll need to for this job, you won't have anything to worry about for your employment future—for twenty years and beyond.